1. Field of the Invention
The present invention relates to an organic electroluminescent display (ELD) device, and more particularly, to an organic electroluminescent display device where deterioration due to bending and protrusion is prevented and a method of fabricating the same.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic ELD devices have been of particular interest in research and development because they have high brightness and low driving voltage. For example, since the organic ELD devices are driven with a low voltage of between DC 5V and DC 15V, a driving circuit may be easily designed and fabricated. In addition, since organic ELD devices are an emissive type, organic ELD devices have a high contrast ratio and a thin profile. Also, organic ELD devices can display images without viewing angle limitations. Further, since organic ELD devices have a short response time of several microseconds (μs), the organic ELD devices have a stable operation property at a low temperature and an advantage in displaying moving images.
Organic ELD devices may be classified into a passive matrix type and an active matrix type according to existence of a switching element. In a passive matrix type organic ELD device where a scan line and a signal line that cross each other to define a pixel region are disposed in a matrix without a switching element, the light of each pixel region may have an instant brightness obtained by multiplying a required average brightness and the number of the scan line since each pixel region emits light only while the corresponding scan line is selected.
In an active matrix type organic ELD device where a scan line and a signal line that cross each other to define a pixel region are disposed in a matrix and a thin film transistor (TFT) as a switching element and a storage capacitor are disposed in each pixel region, the light of each pixel region may have an instant brightness corresponding to a required average brightness since a voltage applied to each pixel region is maintained during a frame due to the TFT and the storage capacitor. As a result, the active matrix organic ELD device may have the required average brightness even with a relatively low voltage as compared with the passive matrix organic ELD device. Accordingly, the active matrix organic ELD device has been widely used due to the advantages of low power consumption, high resolution, and large size.
FIG. 1 is a circuit diagram showing an active matrix type organic ELD device according to the related art. In FIG. 1, a pixel region P of an active matrix type organic ELD device includes a switching thin film transistor (STr), a driving thin film transistor (DTr), a storage capacitor StgC, and an organic electroluminescent diode E. A gate line GL is disposed along a first direction, and a data line DL is disposed along a second direction crossing the first direction. The gate line GL and the data line DL cross each other to define the pixel region P. A power line PL is parallel to and spaced apart from one of the gate line GL and the data line DL. The switching TFT STr is connected to the gate line GL and the data line DL, and the driving TFT DTr is electrically connected to the switching TFT STr. In addition, the driving TFT DTr is electrically connected to the organic electroluminescent diode E and the power line PL. For example, a first electrode of the organic electroluminescent diode E may be connected to a drain electrode of the driving TFT DTr, and a source voltage of the power line PL is transmitted to the organic electroluminescent diode E through the driving TFT DTr. The storage capacitor StgC is formed between a gate electrode and a source electrode of the driving TFT DTr.
When a gate signal is applied to the gate line GL, the switching TFT STr is turned on and a data signal of the data line DL is applied to the gate electrode of the driving TFT DTr. As a result, the driving TFT DTr is turned on and light is emitted from the organic electroluminescent diode E. The grey level of the light emitted from the organic electroluminescent diode E is determined according to the intensity of a current flowing from the power line PL to the organic electroluminescent diode E through the driving TFT DTr. Since the storage capacitor StgC keeps the voltage of the gate electrode of the driving TFT DTr constant while the switching TFT STr is turned off, the constant current flows through the organic electroluminescent diode E during a frame even when the switching TFT STr is turned off.
FIG. 2 is a cross-sectional view showing an organic electroluminescent display device according to the related art. In FIG. 2, an organic ELD device includes first and second substrates 3 and 31 facing each other. The first substrate 3 includes glass and the second substrate 31 is used for encapsulation. A boundary portion of the first and second substrates 3 and 31 is sealed with a seal pattern 40. A driving thin film transistor DTr and a first electrode 12 are formed in each pixel region P on the first substrate 3, and the first electrode 12 is connected to the driving TFT DTr. An organic luminescent layer 14 is formed on the first electrode 12, and a second electrode 16 is formed on the organic luminescent layer 14. The organic luminescent layer 14 includes red, green, and blue emitting material patterns 14a, 14b and 14c emitting red, green, and blue lights, respectively, and the second electrode 16 is formed on the red, green, and blue emitting material patterns 14a, 14b and 14c. An electric field generated between the first and second electrodes 12 and 16 is applied to the organic luminescent layer 14, and the first electrode 12, and the second electrode 16 constitute an organic electroluminescent diode E.
The first and second substrates 3 and 31 are attached to each other by the seal pattern 40, and the second electrode 16 over the first substrate 3 is spaced apart from the second substrate 31. The second substrate 31 includes a groove GR on an inner surface thereof, and an absorbent material 32 such as barium oxide (BaO) or calcium oxide (CaO) is formed in the groove GR to prevent moisture penetration from the exterior. Since the organic luminescent layer 14 deteriorates from exposure to oxygen or moisture, the first substrate 3 including the organic luminescent layer 14 is encapsulated by the second substrate 31 including the absorbent material 32 so that penetration of oxygen or moisture can be prevented.
The absorbent material 32 has a thickness t within a range of about 150 μm to about 250 μm, and the first and second substrates 3 and 31 have a gap distance within a range of about 6 μm to about 12 μm using a glass fiber (not shown) or a spacer (not shown). To keep the gap distance within a range of about 6 μm to about 12 μm, the groove GR having a depth greater than the thickness of the absorbent material 32 is formed in the second substrate 31 and the absorbent material 32 is formed in the groove GR.
When the first and second substrates 3 and 31 are formed to be spaced apart from each other by a gap distance greater than about 150 μm using a thick glass fiber or a thick spacer, the waste of material increases and the thickness of the organic ELD device increases. In addition, since the thick glass fiber or the thick spacer is vulnerable to external impact, the reliability of the organic ELD device is reduced. Further, since a thickness of the seal pattern 40 increases, an area exposed to the exterior increases and penetration of oxygen or moisture increases. Accordingly, the groove GR is formed in the second substrate 31 and the absorbent material 32 is formed in the groove GR.
However, since the groove GR is formed in the second substrate 31, the thickness of the second substrate 31 corresponding to the groove GR is reduced and the second substrate 31 having a reduced thickness is vulnerable to external impact. For example, the second substrate 31 may be broken or cracked during one of the steps of forming the absorbent material 32 in the groove GR, attaching the first and second substrates 3 and 31, or transferring the second substrate 31. In addition, the groove GR is formed to correspond to a display area of the second substrate 31 and the absorbent material 32 is formed in the groove GR. As a result, light of the organic electroluminescent diode E cannot be emitted through the second substrate 32 and the organic ELD device cannot be applied to an upper emission type.